1. Field of the Invention
The present invention relates to a color image reading apparatus and, more particularly, to a color image reading apparatus suitable for, e.g., a color scanner or a color facsimile apparatus, which improves color reproducibility by using color-separating means constituted by a one-dimensional blazed diffraction grating and light-receiving means in which three line sensors are arranged on the surface of the same substrate, thereby reading color image information on the surface of an original with a high precision.
2. Related Background Art
A conventional apparatus for imaging color image information formed on the surface of an original onto the surface of a line sensor, such as a CCD, via an optical system, thereby digitally reading the color image information by using an output signal from the line sensor has been proposed in, e.g., Japanese Laid-Open Patent Application No. 2-214730.
FIGS. 1A and 1B are a plan view (main scanning section) and a side view (sub-scanning section), respectively, showing the main part of a conventional color image reading apparatus. FIG. 2 is a view for explaining a transmission-type one-dimensional blazed diffraction grating shown in FIGS. 1A and 1B.
Referring to FIGS. 1A and 1B, light beams from a color image formed on an original surface 61 are focused by a telecentric imaging optical system 62 and imaged on the surface of a three-line sensor 64 through a transmission one-dimensional blazed diffraction grating 63 for color separation. In this case, the light beams are separated into three colors, for example, blue (B), red (R), and green (G), and light components of these three colors are guided to the surfaces of line sensors 65, 66, and 67 each consisting of a CCD or the like. The color images formed on the surfaces of the line sensors 65, 66, and 67 are line-scanned in a sub-scanning direction, separately reading the individual light components of the respective colors.
In the above conventional color image reading apparatus, as shown in FIG. 2, the grating heights (h.sub.1 to h.sub.3) of a grating 63A of the one-dimensional blazed diffraction grating (to be also referred to as a "diffraction grating" hereinafter) for color separation are constant throughout the whole area of the diffraction grating. For this reason, the spectral characteristics of diffracted light components of the respective orders, spectrally separated by the diffraction grating 63, have waveforms (curves) BO (blue light component), GO (green light component), and RO (red light component) indicated by alternate long and short dashed lines shown in FIG. 3.
Referring to FIG. 3, BO, Go, and RO are set to be the +1st-order, -1st-order, and 0th-order light components, respectively.
Based on the principle of a diffraction grating, the spectral characteristics of these diffracted light components are such that at a peak wavelength .lambda..sub.G of the -1st-order diffracted light component (the spectral distribution of GO shown in FIG. 3), for example, the diffraction efficiencies of the other diffracted light components (the spectral distributions of BO and RO shown in FIG. 3) are very low, nearly 0.
Generally, in order for a color image reading apparatus to improve its color reproducibility and read color images with a high precision, it is desirable that the spectral characteristics of diffracted light components of the respective orders have waveforms B, G, and R with wide wavelength bands indicated by solid lines in FIGS. 3 and 4.
A comparison among the spectral characteristics of the color light components B, G, and R shown in FIG. 4 and those of the conventional color light components BO, GO, and RO shown in FIG. 3 reveals that the spectral characteristics of the color light components BO, GO, and RO which are spectrally separated by the conventional diffraction grating obviously have narrower bands. This gives rise to a problem of poor color separation characteristics: color reproducibility is impaired for some objects to be imaged, such as color photographs (images).
Note that in the one-dimensional blazed diffraction grating 63 shown in FIG. 2, the number of steps is four, each of the step film thicknesses (grating heights) h.sub.1 to h.sub.3 is 848.5 nm, and the angle of incidence of a light beam on the optical axis is 45.degree..
Generally, in performing color reproduction by using a color image reading apparatus, spectral characteristics (spectral distributions) are desirably such that the band width of each of B, G, and R light components is large as shown in FIG. 4. For this reason, widening the wavelength band of each diffracted light component is important in correctly reading color images.